This invention relates generally to flat panel display devices and particularly to a system for refreshing the stored electron gun drive voltages of such a device.
U.S. Pat. No. 4,126,814 issued to F. J. Marlowe discloses a system for uniformly controlling the visual display brightness of a modular flat panel display device. The display device is divided into a plurality of channels, each of which contributes a portion of every horizontal line of the visual output. Such display devices are known in the art as modular display devices. The visual display of a modular device is acceptable only when the display brightness of every channel is uniform within the perception capability of the human eye.
Additionally, the display brightness of the modules must be uniform within approximately 1% to assure that the seams between the modules are invisible. In a modular display device, every channel uses a different electron gun. The electron guns and their associated circuitry typically do not have uniform transfer characteristics and, therefore, the display brightness of the channels will vary along with the transfer characteristic variations. The Marlowe system addresses this problem by storing the brightness drive voltages required to produce 64 different predetermined levels of picture brightness for each of the electron guns. The stored drive voltages are selected from a voltage range which is divided into 256 equal incremental voltages. The electron gun is driven by the incremental voltages and the voltages produced on an electron collector are compared with a reference voltage. The incremental voltage which brings the collector voltage up to the reference voltage is then stored. This procedure is repeated for 64 reference voltages so that 64 of the 256 incremental drive voltages are stored. Accordingly, uniform brightness for all modules in the display is achieved because the brightness levels for all the modules are referenced to the same 64 reference voltages. However, because the electron gun transfer characteristics vary, the drive voltage range from which the 256 incremental voltages are selected must be sufficiently broad to include the electron gun with the greatest cut off voltage at the low end and the gun which requires the most drive to achieve maximum brightness at the high end. In many instances the electron gun characteristics are widely different so that the 256 incremental drive voltage range must be expanded to include both of these guns. As a consequence of this expansion brightness resolution is lost.
U.S. patent application Ser. No. 170,742 filed July 21, 1980 now U.S. Pat. No. 4,344,021 by Loren B. Johnston and entitled "System For Compensating For Transfer Characteristic Variations Of Electron Guns" now U.S. Pat. No. 4,344,021 describes a system which can be used in the Marlowe uniform brightness control system for adjusting the stored electron gun drive voltages. The Johnston system adjusts the stored electron gun drive voltages so that the voltage range is the same for all electron guns. A maximum number of the 256 incremental brightness drive voltages used in the Marlowe system therefore can be utilized for all of the electron guns in the display device.
U.S. Pat. No. 4,167,690 issued to R. A. Gange on Sept. 11, 1979, discloses a method of heating a line cathode in a modular flat panel display device. In the Gange method, a series of discrete heating pulses is passed through the line cathode to cause the cathode to emit electrons. During the intervals between consecutive heating pulses, the entire cathode is at the same potential and therefore during these intervals the display is generated.
In the Marlowe uniform brightness control system the stored electron gun drive voltages are periodically refreshed to correct for thermal drifts of the cathode characteristics. The required refreshing is affected during the vertical retrace time when no display is being generated on the screen. However, in the Marlowe system, the transfer characteristic is swept in a continuous pass. When pulse cathode heating is used, the only continuous time periods are the intervals between the heating pulses. These intervals are insufficient for a continuous refresh of the cathode characteristic and accordingly segmented refresh is required.
The instant invention fulfills this requirement by the provision of a system for sequentially refreshing segments of the stored electron drive voltages so that a preselected number of the drive voltages is refreshed during each of the vertical retrace periods until the entire electron gun transfer characteristic has been refreshed.